The G protein-coupled receptors (GPCRs) comprise a broad family of receptors that activate a large number of effectors in response to a variety of signals that include amines, photons, lipids peptides and proteases. Signaling through GPCRs requires the coordinated balance between processes that govern receptor activation, desensitization and resensitization. Desensitization of GPCRs involves receptor phosphorylation by specific G protein-coupled receptor kinases (GRKs) and interactions with arrestin proteins. GRKs and arrestins desensitize the second messenger signaling function of GPCRs and also initiate the internalization of desensitized GPCRs to re-establish normal responsiveness. In addition, GRKs and arrestins may contribute to novel GPCR signaling paradigms. The overall objective of the proposed research is to define the basic molecular and cellular mechanisms that contribute to these processes with the goal of better understanding physiological and pathological conditions. The dopamine and adrenergic receptors will be used as the prototypic GPCRs since these receptors control a wide variety of function that include vascular tone, cardiac function, and endocrine secretion to complex behaviors such as locomotion, cognition and affect. Aim I: Real-time dynamic assays will be used to follow activation and desensitization of GPCRs and assess the selectivity and structure-function of various members of the GRK and arrestin families. Aim II: The molecular and cellular basis of GPCR internalization and resensitization. as well as the role of GRKs and arrestins in these processes, will also be examined. Aim III: The molecular basis and physiological relevance of the emerging signaling function of the GPCR/GRK/betaarrestin complexes will be examined in cellular and transgenic animal systems. Aim IV: Using the above paradigms and genetically engineered knockout animals, the signaling properties and physiological role of members of the D2 dopamine receptor family will be investigated. The biochemical, pharmacological, physiological, and gene-targeting approaches derived from this work should broaden our understanding of the role of the GPCRs in normal physiology and disease and they may provide new insights into novel therapeutic approaches for treating conditions as diverse as hear failure, drug abuse, and psychiatric disorders.